Fertilization requires the proper interaction between the egg and sperm. Such a targeted event, even under natural conditions, is random, and hence the genetic make up of the nascent genome is unpredictable. The impending pregnancy has to prepare the maternal environment towards acceptance of a semi- or even a total allograft. This preparation can be divided to four distinct phases; the first is the pre-fertilization period, the second is the fertilization/post fertilization, the third phase is trophoblast development, and the final fourth phase is the implantation period.
The first phase, which is the pre-fertilization period, takes place during follicular development. The egg is surrounded by the cumulus oophorus and it is bathed in the rich follicular fluid. This fluid has some immune suppressive activity that may facilitate the fertilization process as well as post-fertilization development. This immune suppressive activity is required, due to the fact that shortly after fertilization, expression of foreign antigens, caused by the sperm, may be present. This mechanism, however, is not a necessary requirement. In cases of in vitro fertilization, no such fluid is available, and fertilization takes place without difficulty in an artificial environment. Moreover, women who have no ovaries can get pregnant by donor embryo transfer.
The next phase is the fertilization/post fertilization process. Once the sperm penetrates the egg it becomes “non visible” by the maternal environment. During the process of egg and sperm head fusion, as long as the egg surface membrane does not change its characteristics and become recognizable by the maternal immune system, no immune reaction would be expected. To safeguard the fertilized egg, it is rapidly surrounded by the zona pellucida, which is a hard and impenetrable shell designed to ward off immune cells. A further protection is due to the presence of the maternal cumulus cells. Those cells may further prevent direct access of immune cells to the embryo. The cumulus cells persist only for the first few days after fertilization because they facilitate the fallopian tube's cilia to propagate the zygote towards the uterus. Following fertilization, it is not excluded that small proteins derived from the maternal environment, such as cytokines, will reach the zygote and early embryo. So far, there has been no evidence for such an occurrence. Following the few initial embryonic cell divisions, to the eight-cell stage, the trophoblast phase is initiated.
The trophoblast phase that occurs by the sixteen-cell stage leads to embryoblast and trophoblast differentiation. While the trophoblast's genome is principally paternally derived, the embryoblast's genome is principally maternally derived. However, since the zona pellucida still surrounds the embryo, it provides a major protection against maternal immune onslaught. Therefore, it appears that the early embryo, during the pert-implantation phase, is rather well protected from maternal immune system. This is despite the fact that the embryo is a semi-antigen. This period in in vitro fertilization/embryo transfer (IVF/ET) procedures does not occur. Consequently, the development of maternal tolerance remains permissive until implantation, which is where direct embryo/maternal contact become a necessary prerequisite.
The final preparation phase is implantation, which occurs when the embryo reaches the uterus and intimate maternal contact is initiated. During implantation the zona pellucida opens, and the trophoblast cells are extruded. This is the time that the embryo is most vulnerable. The embryo is not yet attached to the maternal surface, and it is still exposed to endometrial maternal immune cells as well as potentially hostile cytokines. Of all phases of reproduction, the implantation phase is the most crucial. Specifically, in the case of embryo transfer, following IVF, the embryo has to sojourn in the endometrial cavity for 4-5 days until the maternal organism will accept it. This is a period of endometrial priming by embryonic signaling that leads to maternal tolerance, which is the pre-requisite for successful pregnancy.
Mammalian reproduction was the last to evolve and it required a major shift in the immune system. This is because it allowed a sperm, which might be regarded as a parasite, to invade the maternal organism, and impose, in part, its own genome expression. This suggests that the embryo must have an active role in allowing the initiation of pregnancy. This suggestion is supported by the following observations:
(i) Donor embryos can implant without difficulty, therefore sharing of maternal antigens is not required;
(ii) The site of embryo implantation is not obligatory, although the uterus is preferred. Occasionally, implantation can be found in the fallopian tubes, ovaries, and even inside the abdominal cavity;
(iii) Under certain circumstances, embryos from one species can be implanted and delivered by another species. Genetic mismatch also does not prevent a successful reproduction (i.e.: mule);
(iv) Only viable embryos will implant. Therefore, the implantation is an active act that requires a passive accommodation by the maternal recipient, upon which the embryo can impose its will, in order for the pregnancy to develop;
(v) Sick mammals can also get pregnant, which indicates that the maternal organism does not need to be in good health in order for pregnancy to initiate. This reiterates the passive role of the maternal organism and supports a specific embryo effect;
(vi) The chances of multiple embryos to implant are higher than that of a single embryo. Consequently, an enhanced embryo derived signaling is likely to lead to maternal acceptance; and
(vii) Although a window of opportunity for implantation does exist, it is not strict. Therefore the embryo can implant in less than favorable endometrium, as well.
In conclusion, it appears that the embryo, to a large degree, controls it own destiny. This destiny is irrespective of timing in cycle, site of implantation, the sharing of genes, species, or the health of the mother.
Early work on pregnancy suggested that shortly after fertilization, certain changes that favor tolerance take place in the maternal environment. Those changes were believed to be due to early pregnancy factor (EPF) and platelet activating factor (PAF). However, these factors are not specific to pregnancy and are found in a non-pregnant state as well. More recent work indicates that the embryo's presence, and the products that it secretes, creates a favorable and tolerant environment for a successful pregnancy. Several reports have shown that the embryo-conditioned media has immune-modulatory effects on the maternal organism. The addition of the conditioned culture media, from human and mouse embryos, affects human immune cells activity. This immune modifying activity occurs very early, at the two-cell stage embryo. The activity is dependent on embryo viability, since the media of cultured atretic eggs does not exert such immune-modulatory features. Therefore, shortly after fertilization, the embryo starts actively to emit signals that create maternal recognition of pregnancy which lead to immune tolerance. The cumulus cells, which surround the segregated embryo, may serve as a relay system, since they contain active immune cells that secrete cytokines. Such an intimate contact between putative embryo-derived compounds, and the maternal immune system, would allow for a rapid diffusion of signals from the embryo. This would lead to a local immune response, due to the embryo presence, followed by a systemic maternal immune recognition. Such changes in maternal immunity are shown via a variety of bioassays, including a pre-implantation factor (PIF) assay that measures immune changes in the maternal systemic circulation. This immune change occurs within the first few days after fertilization. Additionally, using IVF cycles, it has been shown that within three days after embryo transfer, PIF activity can be found already in maternal circulation. This indicates that embryo-initiated signaling will rapidly create a systemic immune system tolerance to the embryo. Without wishing to be bound by theory, increased PIF activity may explain why implantation does not necessarily take place in the uterus, but it can occur elsewhere within the organism, and suggest that for embryo transfer to be successful, a similar PIF signal has to exist.